Kinase signaling pathways play a central role in numerous biological processes. Defects in various components of signal transduction pathways have been found to account for a vast number of diseases, including numerous forms of cancer, inflammatory disorders, metabolic disorders, vascular and neuronal diseases (Gaestel et al. Current Medicinal Chemistry (2007) 14:2214-2234). In recent years, kinases that are associated with oncogenic signaling pathways have emerged as important drug targets in the treatment of various diseases including many types of cancers.
Receptor tyrosine kinases (RTKs) are a group of cell surface receptors with high affinity for a variety of polypeptide growth factors, cytokines, and hormones. RTK signaling plays roles in processes such as cell growth, cell survival, proliferation, development and differentiation. Disruption of RTK signaling can lead to diabetes and related complications, neurological disorders such as Alzheimer's disease, cancer, arthritis, inflammatory diseases such as acute coronary syndrome, and autoimmune diseases such as multiple sclerosis.
Approximately 20 different classes of RTKs have been identified, based on homology, including the EGF receptor family (also known as the ErbB family), the Insulin receptor family, the PDGF receptor family, the FGF receptor family, the VEGF receptor family, the HGF receptor family, the Trk receptor family, the Eph receptor family, the AXL receptor family, the LTK receptor family, the TIE receptor family, the ROR receptor family, the DDR receptor family, the RET receptor family, the KLG receptor family, the RYK receptor family, and the MuSK receptor family. RTKs comprise an N-terminal extracellular region, a C-terminal intracellular region that comprises the catalytic kinase domain, and a transmembrane domain. The N-terminal region comprises the ligand-binding region. Upon binding to its ligand, an RTK becomes catalytically active and can phosphorylate itself and activate downstream signaling molecules, including PI3K and Ras. Some RTKs act as a single monomer, while others form dimers or dimerize upon ligand binding.
Another group of kinases involved in cellular functions that are commonly deregulated in diseases is the Phosphatidylinositol 3-kinases (PI 3-kinases or PI3Ks) family of enzymes. These lipid kinases phosphorylate the 3-position hydroxyl group of the inositol ring of phosphatidylinositol (PtdIns), activating signaling cascades associated with such processes as cell growth, proliferation, differentiation, motility, survival and intracellular trafficking. Disruption of these processes involving PI3K leads to many diseases including cancer, allergic contact dermatitis, rheumatoid arthritis, osteoarthritis, inflammatory bowel diseases, chronic obstructive pulmonary disorder, psoriasis, multiple sclerosis, asthma, disorders related to diabetic complications, and inflammatory complications of the cardiovascular system such as acute coronary syndrome.
The PI3K family comprises 15 kinases with distinct substrate specificities, expression patterns, and modes of regulation. The class I PI3Ks (p110α, p110β, p110δ, and p110γ) are typically activated by tyrosine kinases or G-protein coupled receptors to generate phosphatidylinositol-3,4,5-trisphosphate (PIP3), which engages downstream effectors such as those in the Akt/PDK1 pathway, mTOR, the Tec family kinases, and the Rho family GTPases.
The alpha (α) isoform of type I PI3K has been implicated in a variety of human cancers. Angiogenesis has been shown to selectively require the α isoform of PI3K in the control of endothelial cell migration. (Graupera et al, Nature 2008; 453; 662-6). Mutations in the gene coding for PI3K α or mutations which lead to upregulation of PI3K α are believed to occur in many human cancers such as lung, stomach, endometrial, ovarian, bladder, breast, colon, brain and skin cancers. Often, mutations in the gene coding for PI3K α are point mutations clustered within several hotspots in helical and kinase domains, such as E542K, E545K, and H1047R. Many of these mutations have been shown to be oncogenic gain-of-function mutations. While other PI3K isoforms such as PI3K δ or PI3K γ are expressed primarily in hematopoietic cells, PI3K α, along with PI3K β, is expressed constitutively.
The delta (δ) isoform of class I PI3K has been implicated, in particular, in a number of diseases and biological processes. PI3K δ is expressed primarily in hematopoietic cells including leukocytes such as T-cells, dendritic cells, neutrophils, mast cells, B-cells, and macrophages. PI3K δ is integrally involved in mammalian immune system functions such as T-cell function, B-cell activation, mast cell activation, dendritic cell function, and neutrophil activity. Due to its integral role in immune system function, PI3K δ is also involved in a number of diseases related to undesirable immune response such as allergic reactions, inflammatory diseases, inflammation mediated angiogenesis, rheumatoid arthritis, auto-immune diseases such as lupus, asthma, emphysema and other respiratory diseases. Other class I PI3K involved in immune system function includes PI3K γ, which plays a role in leukocyte signaling and has been implicated in inflammation, rheumatoid arthritis, and autoimmune diseases such as lupus.
PI3K β has been implicated primarily in various types of cancer including PTEN-negative cancer (Edgar et al. Cancer Research (2010) 70(3): 1164-1172), and HER2-overexpressing cancer such as breast cancer and ovarian cancer.